Our expedition returned to Volcán Poás
today in hopes of reaching hot springs on the mountain's remote
west side. We entered the cloud forest at the national park's observation
point, peering into the fog
for a glimpse of the acid
lake far below. The rain-washed path beneath the dense canopy
soon led us into the emerald
cloud forest most remarkable for the fact that this entire
mountainside was devastated in the 1953 eruptionan astonishing
example of the process of rapid re-vegetation.

Too soon we left the forest and began the long hike around the crater's
rim. After two hours spent traversing a landscape devastated
by eruptions, we descended the mountain along a cascading river,
and reentered the cloud forest at a lower elevation on the opposite
side of the mountain"the back side," where virgin
forest greeted us like an impenetrable green wall. Here we split
the party into two groups; one returning to the crater's edge in
order to get the photographs you see here today, and the other
proceeding into the bush to search for the elusive hot springs.
The latter group was led by our local expert, Costa Rican volcanologist
Guillermo
Alvarado, who was joined by guide Carlos Ramirez, student
Alison Shaw, and volcanologist Toby Fischer, who sought water samples
from the springs. Toby's report follows:

Today we saw that the majority of the gases are discharging from
the 1953 volcanic dome. These fumaroles are much more vigorous than
what we were able to sample yesterday. The dome is covered with
sulphur, suggesting that at one point the gas temperatures were
at least 135°C. We also observed that the gas plume emanating
from the dome was particularly extensiveand killing
trees in the flanks. This is in drastic contrast to almost
no visible plumes at Irazu and Turrialba. These observations suggest
that the magma,
which is the source of the gases, may be closer to the surface at
Poás compared with the other two
volcanoes. The gas compositions should test this hypothesis.

We expect a higher HCl/CO2 ratio at Poás because
a higher ratio would indicate a more direct pathway for the gases
from the magma to the surface. In contrast, if the ratio is relatively
low, then this may show that the gases interact significantly with
a hydrothermal system and that the magma is at a deeper level.

Previous geophysical research suggests that a small magma chamber
may exist 500 metres below the crater floor of Poás. At Irazu,
geophysical data indicates that magma lies two to five kilometers
below the surface. So, with our gas studies, we hope to confirm
or possibly modify the geophysical interpretations.